The original definition is slowly being replaced by the grime definition. Dunno or Dunn'no is referenced after or during a saying of which the general public are assumed to not knowing - hence hightening your reputation on the mere basis that you know something others dont.

"Dunno the youth... dunno the youth" (pronounced - Dun'No)
"Dunno... Ice Kid" (pronounced - Dun'No)
"I went to see batman the other day" was it good? "yeh dunno" (pronounced - Dun'No) (i.e. you saw it and it was really good)